Methods and systems for reducing crosstalk

ABSTRACT

At least one example embodiment discloses a method of determining crosstalk in a multiple-input-multiple output (MIMO) system. The method includes receiving, from at least one first remote node, upstream pilots on an upstream channel, determining upstream channel coefficients based on the received pilots, transmitting, to the at least one first remote node, downstream pilots on a downstream channel, receiving, from the at least one first remote node, loopback pilots on the upstream channel, the loopback pilots being loopback signals of the downstream pilots, and determining downstream channel coefficients based on the received downstream pilots.

BACKGROUND

Performance of a digital subscriber line (DSL) in terms of capacitydepends on a number of factors such as attenuation and a noiseenvironment. Performance of a DSL transmission system is impacted bycrosstalk interference from one twisted line pair to another twistedline pair with the same binder and, to a lesser extent, twisted linepairs in neighboring binders.

Consequently, crosstalk interference may affect data rates across anumber of twisted pair lines.

For instance two communication lines such as two VDSL2 lines which arecollocated next to each other induce a signal in each other. Due to theinduced crosstalk and noise from other sources in the surroundings ofthe communication line, the data transported on these lines may beaffected or corrupted by the crosstalk and noise. By reducing thecrosstalk induced on a communication line or compensating the crosstalkinduced on a communication line, the amount of corrupted data may bereduced and the rate at which information can be reliably communicatedis increased.

Crosstalk channel compensating filters can be used to reduce the effectsof crosstalk on the communication line or to compensate for thecrosstalk in order to remove the problem almost entirely.

Each communication line is a possible disturber line which inducescrosstalk in one or more victim lines. By transmitting test signalsacross all the lines, it is possible to determine the influence of eachdisturber line on the victim lines. The test signals can becharacterized by the way in which power is allocated to one or moretones or frequencies. For instance, a test signal may be transmittedusing a particular power level over a small frequency range. The victimline may notice this power in that frequency range and be able todetermine the amplitude of that power. The amplitude of the inducedinfluence of crosstalk on a particular line is a good reference todetermine how strong particular crosstalk disturbers are into thatvictim or which frequencies or tones are susceptible to the crosstalk ofcertain crosstalker disturbers into that victim.

Precoding (also referred to as precompensation) techniques are based ontransmitting an additional signal added to the data signal which is usedto compensate the crosstalk on a victim line from external sources.Thus, instead of reducing the effect of crosstalk or avoiding crosstalkeffects by configuring the communication line in an appropriate way,precoding can be used to compensate for the effects of crosstalk on acommunication channel. Precoding techniques are based on crosstalkchannel information that includes both amplitude and phase information.Such information can be obtained from measurements such as slicer erroror signal-to-noise ratio (SNR). A particular example of suchmeasurements for precoding is the use of pilot sequences and errorfeedback. The use of pilot sequences in G.vector is described in“Self-FEXT cancellation (vectoring) for use with VDSL2 transceivers,”Series G: Transmission Systems and Media, Digital Systems and Networks,ITU G.993.5, April 2010, the entire contents of which is incorporated byreference.

SUMMARY

Example embodiments are directed to methods and systems for reducingcrosstalk. Moreover, methods and systems according to exampleembodiments improve data rates across short copper twisted pair linesthat are subject to mutual crosstalk interference. The systems may below cost systems since components may be used in a relativelyhigh-volume networking market.

Example embodiments may be implemented in an environment in whichbroadband access is provided at hundreds of Mbps using fiber optics to adistribution point, followed by copper twisted pair lines less than 200m in length. In other words, example embodiments may be implementedwhere an operator is located close to a subscriber (e.g., within 200 m).

At least one example embodiment discloses a method of determiningcrosstalk in a multiple-input-multiple output (MIMO) system. The methodincludes receiving, from at least one first remote node, upstream pilotson an upstream channel, determining upstream channel coefficients basedon the received pilots, transmitting, to the at least one first remotenode, downstream pilots on a downstream channel, receiving, from the atleast one first remote node, loopback pilots on the upstream channel,the loopback pilots being loopback signals of the downstream pilots, anddetermining downstream channel coefficients based on the receiveddownstream pilots.

In one example embodiment, the upstream pilots are mutually orthogonalto upstream pilots received on other upstream channels.

In one example embodiment, the transmitting the downstream pilotstransmits the downstream pilots after receiving the upstream pilots.

In one example embodiment, the downstream pilots are mutuallyorthogonal.

In one example embodiment, the loopback pilots are amplified by the atleast one remote node.

In one example embodiment, the loopback pilots are at least one offiltered and delayed by the at least one remote node.

In one example embodiment, the method further includes precoding databased on the downstream channel coefficients and transmitting theprecoded data to the remote node.

In one example embodiment, the method further includes receiving datafrom the at least one first remote node and postcoding the data based onthe upstream channel coefficients.

In one example embodiment, the receiving upstream pilots includes,receiving a request to transmit data from one of the at least one firstremote node and receiving the upstream pilots from the at least onefirst remote node and the other first remote nodes.

In one example embodiment, the determining upstream channel coefficientsincludes, determining the upstream channel coefficients based onprevious upstream channel coefficients, the previous upstream channelcoefficients being channel coefficients before receiving the request totransmit data.

In one example embodiment, the method further includes precoding databased on the downstream channel coefficients and transmitting theprecoded data to the at least one first remote node.

In one example embodiment, the method further includes receiving datafrom the at least one first remote node and postcoding the data based onthe upstream channel coefficients.

At least another example embodiment discloses a processor configured to,receive, from at least one first remote node, upstream pilots on anupstream channel, determine upstream channel coefficients based on thereceived pilots, transmit, to the at least one first remote node,downstream pilots on a downstream channel, receive, from the at leastone first remote node, loopback pilots on the upstream channel, theloopback pilots being loopback signals of the downstream pilots, anddetermine downstream channel coefficients based on the receiveddownstream pilots.

In one example embodiment, the processor is configured to transmit thedownstream pilots orthogonally.

In one example embodiment, the processor is configured to precode databased on the downstream channel coefficients and transmit the precodeddata to the remote node.

In one example embodiment, the processor is configured to receive datafrom the at least one first remote node and postcode the data based onthe upstream channel coefficients.

In one example embodiment, the processor is configured to receive arequest to transmit data from one of the at least one first remote nodeand receive the upstream pilots from the at least one first remote nodeand the other first remote nodes.

In one example embodiment, the processor is configured to determine theupstream channel coefficients based on previous upstream channelcoefficients, the previous upstream channel coefficients being channelcoefficients before receiving the request to transmit data.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings. FIGS. 1-5 represent non-limiting, example embodiments asdescribed herein.

FIG. 1 illustrates a conventional DSL system;

FIG. 2 illustrates another conventional DSL system;

FIG. 3A illustrates a system according to an example embodiment;

FIG. 3B illustrates an architecture of the system of FIG. 3A;

FIG. 3C illustrates a TTVP according to an example embodiment;

FIG. 3D illustrates a TTVR according to an example embodiment;

FIG. 3E illustrates a TTVP with echo and NEXT cancellation according toan example embodiment;

FIG. 3F illustrates a TTVR with echo cancellation according to anexample embodiment;

FIG. 4A illustrates a method of reducing crosstalk using a pilotloop-back according to an example embodiment;

FIG. 4B illustrates a stage in the method of FIG. 4A;

FIG. 4C illustrates a stage in the method of FIG. 4A; and

FIG. 5 illustrates a method of reducing crosstalk using a pilotloop-back with NEXT and echo cancellation according to an exampleembodiment.

DETAILED DESCRIPTION

Various example embodiments will now be described more fully withreference to the accompanying drawings in which some example embodimentsare illustrated.

Accordingly, while example embodiments are capable of variousmodifications and alternative forms, embodiments thereof are shown byway of example in the drawings and will herein be described in detail.It should be understood, however, that there is no intent to limitexample embodiments to the particular forms disclosed, but on thecontrary, example embodiments are to cover all modifications,equivalents, and alternatives falling within the scope of the claims.Like numbers refer to like elements throughout the description of thefigures.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of example embodiments. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

It will be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present. Other words used to describe therelationship between elements should be interpreted in a like fashion(e.g., “between” versus “directly between,” “adjacent” versus “directlyadjacent,” etc.).

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of exampleembodiments. As used herein, the singular forms “a,” “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“comprises,” “comprising,” “includes” and/or “including,” when usedherein, specify the presence of stated features, integers, steps,operations, elements and/or components, but do not preclude the presenceor addition of one or more other features, integers, steps, operations,elements, components and/or groups thereof.

It should also be noted that in some alternative implementations, thefunctions/acts noted may occur out of the order noted in the figures.For example, two figures shown in succession may in fact be executedsubstantially concurrently or may sometimes be executed in the reverseorder, depending upon the functionality/acts involved.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which example embodiments belong. Itwill be further understood that terms, e.g., those defined in commonlyused dictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andwill not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

Portions of example embodiments and corresponding detailed descriptionare presented in terms of software, or algorithms and symbolicrepresentations of operation on data bits within a computer memory.These descriptions and representations are the ones by which those ofordinary skill in the art effectively convey the substance of their workto others of ordinary skill in the art. An algorithm, as the term isused here, and as it is used generally, is conceived to be aself-consistent sequence of steps leading to a desired result. The stepsare those requiring physical manipulations of physical quantities.Usually, though not necessarily, these quantities take the form ofoptical, electrical, or magnetic signals capable of being stored,transferred, combined, compared, and otherwise manipulated. It hasproven convenient at times, principally for reasons of common usage, torefer to these signals as bits, values, elements, symbols, characters,terms, numbers, or the like.

In the following description, illustrative embodiments will be describedwith reference to acts and symbolic representations of operations (e.g.,in the form of flowcharts) that may be implemented as program modules orfunctional processes including routines, programs, objects, components,data structures, etc., that perform particular tasks or implementparticular abstract data types and may be implemented using existinghardware at existing network elements or control nodes. Such existinghardware may include one or more Central Processing Units (CPUs),digital signal processors (DSPs),application-specific-integrated-circuits, field programmable gate arrays(FPGAs) computers or the like.

Unless specifically stated otherwise, or as is apparent from thediscussion, terms such as “processing” or “computing” or “calculating”or “determining” or “displaying” or the like, refer to the action andprocesses of a computer system, or similar electronic computing device,that manipulates and transforms data represented as physical, electronicquantities within the computer system's registers and memories intoother data similarly represented as physical quantities within thecomputer system memories or registers or other such information storage,transmission or display devices.

Note also that the software implemented aspects of example embodimentsare typically encoded on some form of tangible (or recording) storagemedium. The tangible storage medium may be magnetic (e.g., a floppy diskor a hard drive) or optical (e.g., a compact disk read only memory, or“CD ROM”), and may be read only or random access. Example embodimentsare not limited by these aspects of any given implementation.

In the context of providing data network access to homes and businesses,various technologies collectively known as FTTx have been used orproposed. In these technologies, data is conveyed from a networkoperator to an intermediate location using fiber optics, and data isconveyed from the intermediate location to the customer location usingDSL transmission over twisted pair copper lines. The term FTTdp refersto a scenario in which the intermediate location is a “distributionpoint”, serving up to a few dozen customers within a distance of lessthan 200 m. The International Telecommunications Union (ITU) has createda working group to develop a recommendation, currently referred to asG.fast, whose purpose is to create a standards-based technology forFTTdp. Example embodiments provide applications such as communicatingdata at high rates (e.g. 200 Mbps) over the twisted pair section of anFTTdp network.

FIG. 1 illustrates a conventional system configured to implement G.fast.As shown, a system 100 includes a distribution point 110 and CustomerPremises Equipment (CPEs) 150 ₁-150 _(n). The distribution point 110 andthe CPEs 150 ₁-150 _(n) may be configured to implement FTTdp. In someexample embodiments, the distribution point 110 may be a DSLAM (digitalsubscriber line access multiplexer). In other example embodiments, thedistribution point 110 may include an individual optical network unit(ONU) associated with each user, and four optical fibers connected. Themultiplexing of the four users into a single line would occur in thefiber optics part of the network. Each of the CPEs 150 ₁-150 _(n) may bein a separate home or office with its own CPE, for example. Moreover,each of the CPEs 150 ₁-150 _(n) may transmit and receive data, andtherefore, may be referred to as transceivers.

The system 100 may be a DSL system, VDSL system or a VDSL2 system, forexample.

The distribution point 110 may be under control of an operator. Thedistribution point 110 includes an ONU 115 configured to communicatewith a network processor (NP) 120. As is known, the ONU 115 provides ahigh-bandwidth data connection over a fiber optic channel to an opticalline terminal (OLT) located in a central office. The ONU passes receiveddownstream data frames or packets to the NP 120, which then determinesthe destination for the frames or packets and accordingly forwards themto an appropriate G.fast interface. Similarly, in the upstreamdirection, the NP 120 forwards frames or packets from the G.fastinterfaces to the ONU 115.

The NP 120 provides signals to processing devices 125 ₁-125 _(n). Eachof the processing devices 125 ₁-125 _(n) provides a G.fast interface.While the number of processing devices 125 ₁-125 _(n) shown is four, thenumber of processing devices 125 ₁-125 _(n) may be greater than or lessthan four. The processing devices 125 ₁-125 _(n) may be physical layersingle-input-single-output (SISO) processing devices. In one example,processing devices 125 ₁-125 _(n) are built for the G.hn (G.9960) homenetworking standard and are adapted for point-to-point communication.The G.hn home networking standard sets forth the communication standardsfor home devices such as set top boxes, laptops and smart power boxes,for example. The G.hn home networking standard allows these home devicesto communicate at high data rates.

Each of the processing devices 125 ₁-125 _(n) may communicate with oneof the CPEs 150 ₁-150 _(n) over communication lines L1-Ln through anassociated line driver (LD) 130 ₁-130 _(n). The communication linesL1-Ln may be twisted line pairs that carry electromagnetic signals. Asshould be understood, the communications are not limited to twisted linepairs. The system 100 may communicate using G.hn signals, VDSL signals,and Ethernet signals, for example. Each pair of processing devices 125₁-125 _(n) and associated LDs 130 ₁-130 _(n) may transmit and receivedata, and therefore, may be referred to as transceivers.

The processing devices 125 ₁-125 _(n) modulate the data, generating atime-domain digital signal to the LDs 130 ₁-130 _(n) consisting of asampled sequence of values to transmit. The LDs 130 ₁-130 _(n) thenconvert the digital signal to analog form, amplify it, and transmit theanalog signal over the communication lines L1-Ln, respectively, to theCPEs 150 ₁-150 _(n), respectively.

FIG. 1 shows a total of four communication lines connected to thedistribution point 110. However, a distribution point may be connectedto a larger number of lines than four. In addition, a precoding groupmay not be limited to the lines connected to a single distributionpoint. A precoding group may for instance contain several tens of lineswhich are distributed over a number of distribution points. In suchcase, coordination between distribution points may be required. FIG. 1furthermore only shows the elements in the communication network whichare relevant for the understanding of example embodiments. Therefore,elements such as network equipment to which the distribution point isconnected, links connecting the distribution point to such equipment,intermediary devices, etc., are not shown in this figure.

While the number of CPEs 150 ₁-150 _(n) shown is four, the number ofprocessing devices CPEs 150 ₁-150 _(n) may be greater than or less thanfour. Each of the CPEs 150 ₁-150 _(n) includes an associated line driver155 ₁-155 _(n) and processing device 160 ₁-160 _(n).

The processing devices 160 ₁-160 _(n) may be the same or substantiallythe same as the processing devices 125 ₁-125 _(n) and, therefore, willnot be described in greater detail for the sake of brevity.

In FIG. 1, the distribution point 110 and the CPEs 150 ₁-150 _(n) areseparated by a distance that is within a communication distance suitablefor high data rate communication using modulation techniques that arethe same as or similar to those set forth in the G.hn home networkingstandard (e.g., within 200 m for communication at 200 Mbps). Therefore,the processing devices 125 ₁-125 _(n) and 160 ₁-160 _(n) may be G.hnprocessing devices. If G.hn processing devices are produced in highvolumes for the home networking market, these processing devices may beless expensive than processing devices specifically designed for theFTTdp market. Consequently, the system 100 has a reduced cost as opposedto processing devices specifically designed for FTTdp.

To adapt the G.hn standard to a point-to-point FTTdp application, somechanges may be required. In G.hn, a single transmission medium is sharedby multiple transceivers, and so transmission of DMT symbols iscoordinated through a medium access control protocol. The loop topologyof an FTTdp scenario would favor scheduling symbol transmissions, and inparticular alternating periods of upstream and downstream transmission.In G.hn, the receivers determine channel coefficients based on preamblesymbols that are sent before each data frame. For FTTdp, it is likelythat channel coefficients would be determined using an initializationprocedure, and maintained through occasional tracking. Nevertheless,much of the core signal processing required to modulate and demodulatedata symbols could be identical.

The communication lines L1-Ln may extend from the distribution point 110to the CPEs 150 ₁-150 _(n), respectively.

However, the system 100 may be subject to crosstalk if the lines L1-Lnare not sufficiently physically separated.

More specifically, any one of the lines L1-Ln may be considered a victimline and the remaining lines L1-Ln may be considered disturber lines.Each of the lines L1-Ln may be associated with a customer. For the sakeof clarity and brevity, L1 will be described as the victim line.

Near end crosstalk (NEXT) is the coupling that occurs between atransmitted signal at one side of a disturber line, for example, thecommunication lines L2-Ln, and a signal at a transceiver (not shown) atthe same end of the victim line L1. For example, coupling betweensignals transmitted from the LD 130 ₂ into the receiver at the LD 130 ₁is near-end crosstalk.

Contrary to NEXT, far end crosstalk (FEXT) occurs, for example, whensignals sent from the distribution point 110 into the disturber linesL2-Ln couple into victim line L1 and cause interference to the receiverat CPE 150 ₁, or signals sent from the CPEs 150 ₂-150 _(n) into thedisturber lines L2-Ln couple into victim line L1 and cause interferenceto the receiver at LD 130 ₁.

In the system 100, data rates could be impacted by crosstalkinterference between the communication lines L1-Ln.

To avoid crosstalk interference, a scheduler may be added to thedistribution point 110. The scheduler allows one subscriber to transmitat a time. An interface would need to be defined between the G.hnprocessing devices 125 ₁-125 _(n) and the scheduler. In this way, eachuser can achieve high peak rates without being affected by crosstalk.

However, when using scheduling, the communication lines L1-Ln become ashared medium, and the average data rates are inversely proportional tothe number of active lines. With a scheduler, there is only one activeline at a time.

To improve performance, vectoring can be used. Vectoring is alsoreferred to crosstalk cancellation. Crosstalk cancellation improves datarates and allows simultaneous communication over multiple lines, insteadof scheduling. Crosstalk cancellation in VDSL2 is described in ITUG.993.5, “Self-FEXT cancellation (vectoring) for use with VDSL2transceivers,” Series G: Transmission Systems and Media, Digital Systemsand Networks, ITU G.993.5, April 2010, also known as the G.vectorrecommendation.

G.vector adds crosstalk cancellation capability to VDSL2. This allowshigh data rates (e.g., many tens of Mbps) across medium distances (e.g.,hundreds of meters). But the limitation of the VDSL2 standard to 30 MHzdoes not allow G.vector to take full advantage of the data rates thatcan be achieved a short distances below 200 m. In other words, thesignals transmitted by VDSL2 devices are only allowed to use frequenciesfrom 0 to 30 MHz. This was not a problem in the past, because higherfrequencies than 30 MHz do not travel very far through twisted paircable. However, now that FTTdp is being considered, the signals don'thave to travel very far since the cables are short, and so frequencieswell above 30 MHz are useful.

FIG. 2 illustrates a conventional system for vectoring (crosstalkcancellation). As shown in FIG. 2, a system 200 includes a distributionpoint 210 and Customer Premises Equipment (CPEs) 250 ₁-250 _(n).

In the system 200, the techniques defined in G.vector are extended to anew physical layer standard that uses a higher transmission bandwidth toget much higher rates. The distribution point 210 may be under controlof an operator. The distribution point 210 includes an optical networkunit (ONU) 215 configured to communicate with an NP 220. The ONU 215 andthe NP 220 are the same as the ONU 115 and the NP 120, respectively, andtherefore, will not be described in greater detail.

The NP 220 provides signals to processing devices 225 ₁-225 _(n). Whilethe number of processing devices 225 ₁-225 _(n) shown is four, thenumber of processing devices 225 ₁-225 _(n) may be greater than or lessthan four. The processing devices 225 ₁-225 _(n) are built for the G.hnhome networking standard and are adapted for point-to-pointcommunication.

The distribution point 210 further includes a frequency domain vectorprocessor (FDVP) 222. The FDVP 222 communicates with the processingdevices 225 ₁-225 _(n).

More specifically, in the downstream direction, the FDVP 222 receivesfrequency domain signal data from the processing devices 225 ₁-225 _(n),applies crosstalk filter coefficients to precompensate the signal dataagainst crosstalk, and provides the processing devices 225 ₁-225 _(n)with the precompensated signal data. For example, assuming that thephysical layer is based on discrete multitone (DMT) signaling, thefrequency domain data could include a representation of a sequence ofcomplex values, representing the constellation points to be transmittedon each of several tones (also known as sub-carriers). The processingdevices 225 ₁-225 _(n) process the precompensated frequency domain datato generate precompensated time-domain signals, which are thencommunicated to the line drivers 230 ₁-230 _(n). In the upstreamdirection, the processing devices 225 ₁-225 _(n) receivecrosstalk-contaminated time-domain signals from the line drivers 230₁-230 _(n), and convert them to crosstalk-contaminated frequency-domainsignal data. The FDVP 222 receives crosstalk-contaminatedfrequency-domain signal data from the processing devices 225 ₁-225 _(n),applies crosstalk filter coefficients to post-compensate the receivedsignals for crosstalk, and provides the processing devices 225 ₁-225_(n) with the post-compensated frequency-domain signal data. Theprocessing devices 225 ₁-225 _(n) then continue to process thefrequency-domain signal data to demodulate the intended upstreaminformation.

The FDVP 222 determines the crosstalk filter coefficients in thefrequency domain in accordance with the pilot-based estimationalgorithms supported by G.vector. The algorithms are based on sendingpilot sequences both upstream and downstream, measuring error signals,forwarding error signals to the FDVP 222, and correlating error signalsagainst transmitted pilots.

Each of the processing devices 225 ₁-225 _(n) may communicate with oneof the CPEs 250 ₁-250 _(n) over the communication lines L1-Ln through anassociated line driver (LD) 230 ₁-230 _(n). For example, the processingdevice 225 ₁ may provide time-domain signals to the LD 230 ₁ for thepurpose of conveying user data to the CPE 250 ₁ over the communicationline L1. In addition to the time-domain signal provided to the LD 230 ₁,the processing device 225 ₁ provides an additional signal to the FDVP222 that is processed based on the crosstalk filter coefficients,together with similar additional signals from processing devices 225₂-225 _(n). After precompensation or postcompensation, the FDVP 222provides a return signal to processing device 225 ₁ that is furtherprocessed by processing device 225 ₁, in a manner that reduces and/oreliminates the crosstalk emanating from the additional lines L2-Ln.

Each of the CPEs 250 ₁-250 _(n) includes a line driver 255 ₁-255 _(n)and a processing device 260 ₁-260 _(n). Each of the line drivers 255₁-255 _(n) may be the same or substantially the same as the line drivers(LD) 230 ₁-230 _(n).

In order to implement the FDVP 222, the processing devices 225 ₁-225_(n) must be configured to enable crosstalk cancellation. In particular,an interface is created that allows the processing devices 225 ₁-225_(n) to communicate frequency-domain data to and from the FDVP 222. Inaddition, the hardware or software controlling the processing devices225 ₁-225 _(n) must provide a mechanism for sending downstream pilotsignals, for estimating error signals relative to upstream pilotsignals, and for forwarding error signals to the FDVP 222. The hardwareor software controlling the processing devices 260 ₁-260 _(n) mustprovide a mechanism for sending upstream pilot signals, for estimatingerror signals relative to downstream pilot signals, and for forwardingerror signals over the upstream communication channel to the processingdevices 225 ₁-225 _(n). The initialization procedures used to establishnew communication sessions between processing devices 225 ₁-225 _(n) andprocessing devices 260 ₁-260 _(n) must be modified to avoid disruptionof active communication sessions between these processing devices.Because of all of the special features needed to support vectoring, theprocessing devices 225 ₁-225 _(n) and 260 ₁-260 _(n) need to besubstantially different than the processing devices 125 ₁-125 _(n) and160 ₁-160 _(n), shown in FIG. 1. Consequently, it is unlikely that theprocessing devices 225 ₁-225 _(n) can be reduced in cost by sharinghigh-volume production with processing devices for the home networkingmarket. Consequently, the vectoring-capable system 200 can be expectedto be much more expensive than the system 100.

FIG. 3A illustrates a system for cancelling crosstalk according to anexample embodiment. In FIG. 3A, a system 300 cancels crosstalk throughvectoring, in a transparent way that does not require any changes toG.hn processing devices. That is, the G.hn processing devices can beused in scenarios with and without crosstalk. For crosstalkenvironments, devices are inserted between the digital and analogportions of the signal chain; these devices perform vectoring,independently of a physical layer operation, and create a channel thatis effectively crosstalk free. This approach is referred to here astransparent time-domain vectoring (TTV). Here, a channel is referred toas a system mapping input signals to output signals over a medium in away that enables communication.

The system 300 is the same as the system 100, in FIG. 1, except adistribution point 310 includes a time domain vector processor (TTVP)327 and CPEs 350 ₁-350 _(n) include time domain vector repeaters (TTVR)357 ₁-357 _(n). The TTVP 327 may simply be referred to as a processor.For the sake of brevity, only the differences will be described.

As shown in FIG. 3A, the TTVP 327 is in the communication path betweenthe processing devices 125 ₁-125 _(n) and the line drivers 130 ₁-130_(n).

In FIG. 3A, inputs and outputs of the TTVP 327 and the TTVRs 357 ₁-357_(n) are digital, time-domain samples.

The TTVP 327 precompensates (precodes) downstream signals andpost-compensates upstream signals. Moreover, the TTVP 327 sends andreceives pilot signals to and from the CPEs 350 ₁-350 _(n) to determinethe filter coefficients needed for the crosstalk cancellation function.The TTVP 327 estimates the upstream channels and associated filtercoefficients based on pilot signals sent upstream by the TTVRs 357 ₁-357_(n), as further described below. The TTVP 327 estimates the downstreamchannels based on pilot signals sent downstream by the TTVP 327 and thenlooped back upstream by the TTVRs 357 ₁-357 _(n). Consequently, the TTVP327 may provide information to a processing device that does not need tobe designed for crosstalk cancellation. In other words, crosstalkbetween high volume G.hn processing devices, another type of SISOprocessing device, may be cancelled with the use of the TTVP 327 and theTTVRs 357 ₁-357 _(n). The TTVP 327 determines crosstalk filtercoefficients using pilot signals, as is described below.

The pilot signals transmitted by the TTVP 327 and the TTVRs 357 ₁-357_(n) may be orthogonal frequency-division multiplexing (OFDM) signals.

The TTVP 327 prevents signals from the processing devices 125 ₁-125 _(n)from reaching the line drivers 130 ₁-130 _(n) until the filtercoefficients are determined. By preventing the signals from reaching theline drivers 130 ₁-130 _(n) until the filter coefficients aredetermined, operation of active sessions on other communication lines isprotected from interference. For example, if active sessions are inplace on the LDs 130 ₂-130 _(n), the TTVP 327 may prevent signals fromthe processing device 125 ₁ from reaching the LD 130 ₁ until the filtercoefficients that cancel crosstalk from the line L1 into the lines L2through Ln, and from the lines L2 through Ln into the line L1, aredetermined.

The TTVRs 357 ₁-357 _(n) send and receive pilot signals used by the TTVP327 to determine crosstalk filter coefficients needed for thecancellation function, which is described in greater detail below. Itshould be understood that the TTVRs 357 ₁-357 _(n) are the same. Thus,where possible, only one TTVR will be described for the purposes ofclarity and brevity.

The TTVRs 357 ₁-357 _(n) prevent physical layer signals from reachingthe LDs 130 ₁-130 _(n) until the TTVP 327 indicates that the filtercoefficients are correct. In a normal operation, the TTVRs 357 ₁-357_(n) receive values as input and repeat them as output. To preventsignals from reaching the LDs 130 ₁-130 _(n), the TTVRs 357 ₁-357 _(n)and TTVP 327 provide zero values as output instead. The TTVP 327 maycommunicate that the filter coefficients are correct over a controlchannel between TTVP 327 and the TTVRs 357 ₁-357 _(n). For example, theTTVP 327 and a TTVR 357 ₁-357 _(n) could use differential phase-shiftkeying (DPSK) signaling at a particular frequency. If a narrow bandwidthis used, the interference between data channels and the control channelcan be limited. In an example system, data may be modulated using 4096DMT subcarriers with 16 KHz subcarrier spacing, while the controlchannel could use DPSK signaling taking up approximately 1 KHzbandwidth.

The TTVP 327 includes a precoder to precode downstream signals for theCPEs 350 ₁-350 _(n). The precoder is frequency dependent. Thus, the TTVP327 is configured to transform time domain samples into the frequencydomain for the precoder and then transform the precoded data from thefrequency domain into time domain samples. The precoder may beimplemented, for example, by an overlap-and-add Fast Fourier Transform(FFT) method.

In the overlap-and-add FFT method implemented by the precoding filter, Ttime samples are zero padded by W samples, an (T+W)-point FFT is appliedto obtain (T+W) Fourier components. This procedure is carried out inparallel for the signals from n active lines. Then, for each Fouriercomponent, the vector of n values from the n active lines is multipliedby an n by n matrix to obtain a precompensated (precoded) vector(downstream) or post-compensated vector (upstream). Next, inverse (T+W)point FFTs are applied to the result on each line, resulting in T+W timesamples. The first T samples are added to W saved values from theprevious iteration to produce T output samples, and the remaining Wvalues are saved to be used in the next iteration. In choosing theparameters W and T, the parameter W should be large enough to cover aperiod of time greater than the delay spread of the direct and crosstalkchannels. The parameter T should be large enough to keep the relativewindowing overhead (T+W)/T sufficiently small. The FFT size T+W does notneed to be the same as the FFT size used for DMT modulation by theprocessing devices 125 ₁-125 _(n), and advantageously could be muchsmaller in some cases.

For upstream pilots, the TTVRs 357 ₁-357 _(n) temporarily ignoreincoming physical layer data from the processing devices 160 ₁-160 _(n),and send instead time-domain pilot samples. The upstream pilot signalsfrom a given TTVRs 357 ₁-357 _(n) will propagate to all of the FTTdp LDs130 ₁-130 _(n), either by the direct channel or by crosstalk. Forexample, the TTVR 357 ₁ receives physical layer samples from theprocessing device 160 ₁. The TTVR 357 ₁ would normally convey thephysical layer sample to the LD 155 ₁. During the upstream pilotoperations, instead of conveying the physical layer sample, the TTVR 357₁ inserts time-domain pilot sample.

The TTVP 327 receives the time-domain signals from the LDs 130 ₁-130_(n) that result from the pilot signals inserted into the upstreamchannel by each TTVR 357 ₁-357 _(n). The received pilots are used by theTTVP 327 to estimate the upstream direct channel and crosstalk channelcoefficients. While pilot signals are being inserted by the TTVRs 357₁-357 _(n), the TTVP 327 may pass on to the physical layer the resultingsamples, samples with value zero, or random noise. This appears to anupstream physical layer receiver (e.g., the processing device 125 ₁) asa temporary loss of signal or impulse noise

For downstream pilots, the TTVP 327 temporarily ignores incomingphysical layer signals from the processing devices 125 ₁-125 _(n), andinstead, sends time-domain pilot samples.

For example, each processing device 125 ₁-125 _(n) sends out a sequenceof numbers at a certain clock rate; a different number at the end ofeach clock cycle. The TTVP 327 reads those numbers in at the same rate,and produces numbers on the other side which go to the corresponding LD130 ₁-130 _(n) at the same rate. Likewise for upstream, the LD 130 ₁-130_(n) sends one number to the TTVP 327 every clock cycle, and the TTVP327 sends one number to a given processing device 125 ₁-125 _(n) inevery clock cycle. These numbers may be referred to as physical layersignals.

In an example, the TTVP 327 receives a physical layer signal sample fromthe processing device 125 ₁. Instead of conveying the physical layersample to the LD 130 ₁, the TTVP 327 inserts a time-domain pilot signal.The TTVP 327 is configured to replace physical layer samples withtime-domain pilot samples for all channels (e.g., between TTVP 327 andall LDs 130 ₁-130 _(n)) in the system 300.

The TTVR 357 ₁ receives the time domain signals from the LD 155 ₁resulting from the pilot signals inserted by the TTVP 327 into the LD130 ₁ and crosstalk from the pilot signals inserted by the TTVP 327 intothe LDs 130 ₂-130 _(n). Based on the received pilot signals, the TTVR357 ₁ provides feedback information received from the received pilotsignals. In at least one example embodiment, the TTVR 357 ₁ processesthe received pilot signals to estimate downlink channel coefficients,and sends the results to the TTVP 327 through a communication channel.

The term feedback, as used in this document, refers to a means by whichthe transceiver of a communication system such as a CPE communicates toa transceiver of the communication system such as a distribution pointvalues derived from received pilot signals.

In another example embodiment, the TTVR 357 ₁ loops-back the receiveddownstream pilot signals in the upstream direction, with amplification,enabling the TTVP to form its own estimated channel coefficients. Inanother example embodiment, the loop-back may be without amplification.The pilot signals and loop-back pilots appear as loss of signal or asimpulse noise to the downstream and upstream physical layer receivers(processing devices 125 ₁-125 _(n) and 160 ₁-160 _(n)). Note that theloop-back approach is more appropriate for a time-division duplex (TDD)rather than frequency-division duplex (FDD) system, since the loop-backsignal sent upstream occupies similar frequency bands as the downstreamsignal. This TDD approach is described in greater detail with referenceto FIG. 4A, which is described in detail below.

As shown in FIG. 3B, the system 300 is organized conceptually intolayers, including a TTV layer 3000, 3100, in addition to physical (PHY),Ethernet and Internet Protocol (IP) layers. The TTV layer 3000 may be alayer associated with a TTVP (e.g., TTVP 327) and the TTV layer 3100 maybe associated with a TTVR (e.g., TTVR 357 ₁).

The architecture shown in FIG. 3B uses in-band communication between theTTVP and the TTVR. The communication is in-band because the TTV layer3000, 3100 operates even when there is no higher layer communicationsession established. The following downstream messages are communicatedfrom the TTVP to the TTVR: (1) Request pilot signal, with specified sign(positive or negative), at specific time, (2) Request loop-back signal,at specified time, (3) Instruct TTVR to begin relaying US/DS samples(for example, after precoder/postcoder training is complete), and (4)Instruct TTVR to stop relaying US/DS samples (for example, as part of anorderly shut-down procedure). Additionally, parameters of loop-backamplification filter may be defined in the downstream messages.

The following upstream messages are communicated from the TTVR to theTTVP: (1) Indicate readiness to create a connection and (2) Indicateimminent shut-down of a connection. Additionally, acknowledgement ofreceipt of various downstream messages may be included in the upstreammessages.

In an example embodiment, a signaling channel between TTVP and TTVRimplements a tone based approach, where the TTVP and TTVR transmit apilot tone signal on pre-specified downstream and upstream frequencies.The pilot tone signal is added on top of any other signal passingthrough. The TTVP communicates bits by inverting the pilot tone, at alow bit rate (i.e. using binary DPSK). This creates the downstream (DS)vectoring operations channel (VOC). Similarly the upstream pilot tone ismodulated to create an upstream (US) VOC. When the TTVR is able todetect the DS VOC, the TTVR echoes the received bits, providing anacknowledgement of correct reception, in an example embodiment. When theTTVR does not detect the DS VOC, the TTVR sends a pre-specified idlepattern. When the TTVR is shut down or is about to shut down, the USpilot tone disappears. Thus, the presence or absence of the US pilottone, and the echoed bits, provides the US messages.

In an example embodiment, downstream messages are sent by the TTVP in aformat using the DS VOC. For example, a pre-specified bit sequenceindicates a request to send a pilot signal. The TTVR acknowledges therequest in the echoed bits on the US VOC. The pilot signal is then sentby the TTVR at a specified time-lag after the echoed message isreceived.

The VOC operates in the presence of interference from physical layertransmissions, crosstalk from other VOC channels (during TTVinitialization), and from external noise including RFI. These forms ofinterference could be mitigated as follows, (1) the physical layersignal may be notched to avoid interfering with VOC (it will have lowSNR at VOC frequencies anyway.) Even if the physical layer is notnotched, the VOC can achieve some SNR gain by averaging over a long timewindow because the VOC modulation is much slower than the physical layermodulation.

As should be understood, “notched” means that a signal is filtered orgenerated in such a way that it has little or no energy near a certainfrequency. In a plot of signal energy versus frequency, the plot has anotch (dips abruptly) at the specified frequency.

FIG. 3C illustrates a TTVP according to an example embodiment.

FIG. 3C illustrates a portion 327 a of the TTVP 327. For simplicity,FIG. 3C illustrates the portion 327 a for vectoring with 2 lines. Itshould be clear how to modify the diagram for arbitrary n.

At the top left is the interface for receiving downstream time domainsamples generated by two processing devices 125 ₁ and 125 ₂. At the topright, is the interface by which downstream time domain samples are sentto LDs 130 ₁ and 130 ₂. At the bottom right is the interface by whichupstream time domain samples are received from LDs 130 ₁ and 130 ₂. Atthe bottom left are interfaces by which upstream time-domain samples aresent to processing devices 125 ₁ and 125 ₂.

In a downstream operation, each serial-to-parallel converter 328 a, 328b collects time domain samples from the associated processing device 125₁, 125 ₂, into batches of length T, and zero pads to a vector of lengthT+W. Fast Fourier Transform units 330 a, 330 b apply the FFT to therespective vectors. Each selector 332 a, 332 b passes along the FFTcomponents during normal operation. At other times, the selector 332 a,332 b may insert zero values, or frequency-domain pilot signalsP_(k)(f,s) coming from an associated pilot generator 334 a, 334 b basedon the mode of operation (e.g., channel estimation) of the TTVP 327.

A precoder 336 performs 2×2 matrix multiplications on the Fouriercomponents. Inverse FFT units 338 a, 338 b transform the output from theprecoder 336 back to respective time domain vectors of length T+W. Eachparallel-to-serial unit 340 a, 340 b takes an output vector of lengthT+W and generates T time-domain output samples, using the overlap andadd method. Finally, control channel transmitters 342 a, 342 b may addtime-domain control channel signals onto output signals from theparallel-to-serial units 340 a, 340 b, respectively, in order to sendcontrol channel messages to the TTVRs 357 ₁-357 ₂, respectively.

In an upstream operation, serial-to-parallel converters 328 c, 328 dcollect time domain samples into batches of length T, and zero pad torespective vectors of length T+W. Fast Fourier Transform units 330 c,330 d apply the FFT to the vectors output from the serial-to-parallelconverters 328 c, 328 d, respectively. A postcoder 344 performs 2×2matrix multiplications on each of the Fourier components produced by theFFT units 330 c, 330 d. Selectors 332 c, 332 d receive zero values andthe output of the postcoder 344. The selectors 332 c, 332 d output zerovalues or the output of the postcoder 344 based on the operation of theTTVP 327, as directed by a controller 348. Alternatively, the selectors332 c, 332 d may pass another value indicating an unavailable signal tothe processing devices 125 ₁ and 125 ₂.

The selectors 332 c, 332 d may be used, for example, to prevent pilotsignals sent upstream from the TTVRs 357 ₁-357 ₂ to the TTVP 327 frompropagating to the processing devices 125 ₁ and 125 ₂. In cases wheresuch pilot signals are not expected to adversely affect the operation ofthe processing devices 125 ₁ and 125 ₂, the selectors 322 c, 322 d maybe omitted.

Inverse FFT units 338 c, 338 d transform back the output from thepostcoder 344 to a time domain vector of length T+W. Input to the IFFT338 c corresponds to the time domain samples from the LD 130 ₁ and inputto the IFFT 338 d corresponds to the time domain samples from the LD 130₂.

Parallel-to-serial units 340 c, 340 d take output vectors of length T+Wfrom the IFFT units 338 c, 338 d, respectively and generate Ttime-domain output samples, using the overlap and add method. A copy ofthe signal received from each LD 130 ₁, 130 ₂ is replicated and passedto an associated control channel receiver 342 c, 342 d, to decodecontrol messages from the TTVRs 357 ₁ and 357 ₂.

Copies of postcoder output symbols are forwarded to a channel estimator346. During times that pilot sequences or delayed loop-back sequencesare being transmitted by the TTVRs 357 ₁-357 _(n) over the upstreamchannel, the channel estimator 346 correlates postcoder output symbolswith pilot symbols to obtain estimates of the resultant crosstalkchannel coefficients. Using knowledge of the resultant crosstalk channeland of the existing precoder 336 and the postcoder 344 filtercoefficients as explained in further detail below, the channel estimator346 determines new precoder and postcoder filter coefficients that areapplied to data from the processing devices 125 ₁-125 _(n) and from theLDs 130 ₁-130 _(n).

A controller 348 determines which operation the TTVP 327 is to perform.For example, the controller 348 determines whether to perform channelestimation or operate in a normal filtering mode. Therefore, thecontroller 348 coordinates the activities of all the features shown inFIG. 3C, determines when to send pilots, when to apply updated precoderand postcoder coefficients, and when to apply zeroes to the selectors332 a-332 d.

FIG. 3D illustrates a TTVR according to an example embodiment. Morespecifically, FIG. 3D illustrates an example embodiment of the TTVR 357₁. Since the TTVRs 357 ₁-357 _(n) are the same or substantially similar,only a description of the TTVR 357 ₁ will be provided for the sake ofbrevity.

In the downstream direction, the TTVR 357 ₁ normally passes time-domainreceived samples directly to the output. A copy of the received signalis sent to a control channel receiver 360, to decode control messagesfrom the TTVP 327. Another copy of the received signal is forwarded toan amplification, filtering, and delay unit 362 during a loopbackoperation. A selector 361 receives zero values and the received signal.The selector 361 outputs zero values or the received signal based on themode of operation of the TTVR 357 ₁.

The selector 361 operates based on commands from a controller 370. Whenthe controller 370 indicates that the TTVR 357 ₁ is in a normal mode ofoperation, the selector 361 passes the received samples directlydownstream. When the controller 370 indicates a loopback operation, theselector 361 may insert zero values to prevent pilot signals sentdownstream from the TTVP 327 to the TTVR 357 ₁ from propagating to theprocessing device 160 ₁. Alternatively, the selector 361 may passanother value indicating an unavailable signal to the processing device160 ₁.

In cases where such pilot signals are not expected to adversely affectthe operation of the processing device 160 ₁, the selector 361 may beomitted.

The amplification, filtering and delay unit 362 may simply pass alongthe received signal to an output with some specified delay, or theamplification, filtering and delay unit 362 may additionally perform afiltering operation. For example, the amplification, filtering and delayunit 362 may convolve the received signal with a fixed set of filtercoefficients, implementing a Finite Impulse Response (FIR) filter.Alternatively, using a recursive filter implementation, theamplification, filtering and delay unit 362 may obtain an InfiniteImpulse Response (IIR) filter. The filters mitigate the effects offrequency-dependent attenuation in the downstream channel, so that theoutput of the amplification, filtering and delay unit 362 hassignificant energy on all frequencies.

The signal sent over twisted pair lines experiences frequency-dependentattenuation, and in particular higher frequencies are more stronglyattenuated. In a loop-back estimation according to an exampleembodiment, then the pilot signal is attenuated twice, once when passingdownstream through the line, and once while passing back upstream. Thismay lead to strong attenuation and poor estimation performance,especially for the highest frequencies. To improve estimationperformance, the TTVR 357 ₁ amplifies the signal during loop-back. Toimprove amplification without violating power spectral densityconstraints, the frequency-dependent amplification is (at leastapproximately) equal and opposite to the frequency-dependent attenuationof the twisted pair line.

To accomplish the frequency-dependent amplification, the amplification,filtering and delay unit 362 may include a plurality of finite impulseresponse filters, each finite impulse response filter being designed toapproximately undo the attenuation of a different length of twisted pairline. For example, the plurality of finite impulse response filters maybe used for lengths of 50 m, 100 m, 150 m, and 200 m, respectively, andmay be designed using well-known numerical optimization techniques.

The TTVR 357 ₁ may store corresponding filter coefficients. For example,the numerical optimization could be chosen, for a given filter length,to make the product of channel and filter have as close to unit gain aspossible, under the constraint that the gain not exceed unity at anyfrequency. Based on an upstream pilot, the TTVP controller 346 decideswhich loopback filter of the amplification, filtering, and delay unit362 will be effective at amplifying the loopback signal withoutexceeding power constraints. The TTVP 327 can then instruct the TTVR 357₁, over the control channel, which filter to use.

The chosen filter is applied by convolving the sequence of stored filtercoefficients with the received pilot signal.

In the upstream direction, the TTVR 357 ₁ passes time-domain receivedsamples into a selector 364. The selector 364 operates based on commandsfrom the controller 370. When the controller 370 indicates that the TTVR357 ₁ is in a normal operation, the selector 364 passes the receivedsamples directly to the output. When the controller 370 indicates aloopback operation, the selector 364 may insert zero values, or mayinsert time-domain pilot values from a pilot generator 366, or mayinsert amplified, filtered, and delayed values obtained from theamplification, filtering and delay unit 362. Finally, a control channeltransmitter 371 may add a time-domain control channel signal onto theoutput signal from the selector 364, in order to send control channelmessages to the TTVP 327.

The controller 370 is responsible for coordinating the activities of allother units in the TTVR 357 ₁-357 _(n), for determining when to sendpilots, and what parameters to use for the amplification and filtering.

FIG. 3E illustrates a TTVP with NEXT cancellation according to anexample embodiment. FIG. 3E illustrates a portion 327 b of the TTVP 327.The portion 327 b is the same as the portion 327 a, except the portion327 b includes a NEXT canceller 349 coupled to the outputs of theprecoder 336 and the inputs of the postcoder 344. Moreover, thecontroller 348, the transmitters 342 a, 342 b and receivers 342 c, 342 dare included in the portion 327 b, but are omitted from FIG. 3E forclarity.

FIG. 3F illustrates a TTVR with echo cancellation according to anexample embodiment. FIG. 3F illustrates a TTVR 357 _(1a). The TTVR 357_(1a) is the same as the TTVR 357 ₁, except the TTVR 357 _(1a) includesan echo canceller 372 coupled to a downstream input and an upstreamoutput (output of the selector 364). Moreover, the controller 370 isincluded in the TTVR 357 _(1a), but is omitted from FIG. 3F for clarity.

The NEXT canceller 349 may implement filters designed to eliminatenear-end crosstalk (NEXT) (interference between different transceiversat or near the same location) as well as echo (interference fromtransmitter to receiver within the same transceiver). The echo canceller372 may implement a filter designed to eliminate echo. The cancellationof NEXT and echo allows full-duplex communication between thedistribution point 310 and the CPE 350 ₁.

A frequency-domain implementation of the NEXT canceller 349 is shown inFIG. 3E, and a time-domain implementation of the echo canceller 372 isshown in FIG. 3F.

In FIG. 3E, copies of the precoder 336 output signals are passed to theNEXT canceller 349. The NEXT canceller 349 applies filter coefficientsto the precoder 336 output signals to obtain a cancellation signal thatis added to the upstream postcoder 344 inputs. The resulting signals arefed back to the NEXT canceller 349, for use in training the filtercoefficients. In an alternative embodiment, a time-domain NEXT cancellercould operate on time domain samples coming out of theparallel-to-serial units 340 a, 340 b.

In FIG. 3F, copies of TTVR 357 _(1a) output signals are passed to theecho canceller 372. The echo canceller 372 applies filter coefficientsto these signals to obtain a cancellation signal that is added to adownstream TTVR input signal. The resulting signals are fed back to theecho canceller 372, for use in training the filter coefficients.

FIG. 4A illustrates a method of reducing crosstalk using a pilotloop-back.

In general, crosstalk cancellation across communication lines isperformed through vectoring. Precoding and postcoding are performed atthe operator side of the distribution point 310, where all lines areterminated.

In order to achieve downstream crosstalk cancellation, a process ofestimation is used by the distribution point 310 to determine thecorrect precoder coefficients (filter crosstalk coefficients). Thedetermination of precoder coefficients is performed by sending a pilotsignal in the downstream direction, and then providing a feedback overan upstream communication channel.

The example embodiment of FIG. 4A provides a method that estimates thedownstream crosstalk channel with minimal signal processing capabilitiesat the customer side (CPEs 350 ₁-350 _(n)). In particular, the CPEs 350₁-350 _(n) have the ability to send signals upstream, but there may beno digital communication established between distribution point 310 andthe CPEs 350 ₁-350 _(n) (or where only a minimal, low-bandwidthsignaling channel is established). In the example embodiment of FIG. 4A,the distribution point 310 first estimates an upstream channel and thena downstream channel.

In one conventional manner, a distribution point (operator side) sends adownstream pilot signal to a CPE (customer side). The CPE uses thereceived pilot to estimate downstream channel coefficients. Theestimated downstream channel coefficients are quantized and sentupstream over an existing digital communication channel. However, thisconventional manner requires an upstream digital communication sessionto be established before the estimation takes place. The session needsto have reasonably high data rate in order for the communication to takeplace quickly.

In another conventional manner, a CPE sends an upstream pilot signal toa distribution point. The distribution point uses the received pilot toestimate upstream channel coefficients. Downstream channel coefficientsare derived from the upstream coefficients using channel reciprocityfeature of isotropic transmission media. However, reciprocity does notnecessarily hold for twisted pair channels, particularly in the presenceof bridged taps and other non-ideal topologies.

In the G.vector standard, a distribution point sends a downstream pilotsignal. The CPE measures and quantizes the difference between receivedpilot signal and the transmitted signal (the error signal). Quantizederror measurements are sent upstream over an existing digitalcommunication session. The distribution point estimates downstreamchannel coefficients based on quantized error measurements. However,G.vector requires an upstream digital communication session withreasonable high data rate (so that initialization does not take a longtime).

In example embodiments, at least one purpose of using G.hn processingdevices is to establish a high-data rate digital communication sessionbetween the distribution point 310 and the CPEs 350 ₁-350 _(n). Binarydata located at one side may be communicated reliably to the other side.

Moreover, the upstream and downstream channels are physical entities. Ifa certain changing voltage is input to one of the upstream anddownstream channels, a resulting voltage is generated at an output. Inat least one example embodiment, a loop-back channel is created byconcatenating the downstream and upstream channels. More specifically,the loop-back channel is created by the system 300 configured to havethe TTVRs 357 ₁-357 _(n) repeat back what they receive, which may beafter amplification or filtering. In other words, the loop-back channelis created with processing devices, which takes the digital samplescoming from the LDs 155 ₁-155 _(n), optionally performs processing inthe time domain, optionally further delays the samples, and sendscorresponding digital samples back to the LD 155 ₁-155 _(n) to sendupstream.

As will be described with reference to FIG. 4A, in at least one exampleembodiment, a CPE amplifies and loops back the received downstreampilots into the upstream direction. A distribution point can thenestimate the downstream crosstalk from the received loop-back signal,based on knowledge of the upstream channel obtained separately fromupstream pilots. The loop-back approach assumes a TDD or full-duplexrather than FDD system because the pilot signals cover all usedfrequencies, both upstream and downstream. More specifically, it is notnecessary that the data communications use all frequencies in bothdirections, but the devices are permitted to send pilot signals on allfrequencies in both directions during the estimation process.

To aid in the description of the method of FIG. 4A, FIGS. 4B-4C areused. Moreover, the method of FIG. 4A is described with reference to thesystem 300. However, it should be understood that the method of FIG. 4Amay be performed by systems different than the system 300.

As shown, in FIG. 4A, the method starts at S400. At the start, somecommunication sessions may be active. For example, FIG. 4B illustratesthe system 300 where three communication sessions are active. Morespecifically, communication channels between the processing devices 125₁-125 ₃ and 160 ₁-160 ₃ are active. One active communication channel isrepresented by the solid line path from G.hn processing device 125 ₁ atthe distribution point, through the TTVP 327, through the LD 130 ₁,across the twisted pair line, through another LD 155 ₁, through the TTVR357 ₁, and finally to the G.hn processing device 160 ₁ in the CPE 350 ₁.Similarly, solid line paths represent active communication channelsbetween processing devices 125 ₂ and 160 ₂, and between 125 ₃ and 160 ₃.As should be understood, the communication channels over twisted pairlines cause crosstalk which are represented by the dashed lines betweenthe distribution point 310 and the CPEs 350 ₁-350 _(n) in FIG. 4B. Thesolid and dashed lines within the TTVP 327 represent 3×3 precoder andpostcompensation matrices that are determined by the TTVP 327 based onestimation of the crosstalk channels between the line drivers 130 ₁-130₃ and 155 ₁-155 ₃, and which are designed to mitigate the effects ofthis crosstalk. More specifically, each downstream and upstreamtransmission over a twisted pair line causes crosstalk with othertransmissions on the remaining twisted pair lines. Consequently, at thisstage, the TTVP 327 implements a 3×3 precoder and a 3×3 postcompensationfilter.

For example, with narrowband channels at a particular frequency f, theupstream channel is represented by a complex matrix

H _(U)=(I+G _(U))D _(U)  (1)

where Gu is a normalized upstream crosstalk matrix and D_(U) is anupstream diagonal matrix representing the diagonal elements of Hu and Iis an identity matrix (a matrix with 1 on the diagonal, and alloff-diagonal elements being zero).

The downstream channel is

H _(D) =D _(D)(I+G _(D))  (2)

where G_(D) is a normalized downstream crosstalk matrix and D_(D) is adownstream diagonal matrix of direct gains. As should be understood, theupstream and downstream channels are multiple-input-multiple-output(MIMO channels).

Filter coefficients in the postcoder 344 and the downstream precoder 336are represented by matrices C_(U) and C_(D), respectively. The notationC will represent the filter coefficients at various points in time. TheTTVP 327 determines C based on estimates of crosstalk channels anddetermines C based on activation/deactivation events and/orperiodically.

Referring back to FIG. 4A, the distribution point 310 receives theupstream pilot signals from the TTVRs 357 ₁-357 _(n) at S410. Morespecifically, when the CPE 350 _(n), not currently having an activecommunication session with the distribution point, requests to transmitdata, the TTVRs 357 ₁-357 _(n) may temporarily prevent data beingtransferred from the processing devices 160 ₁-160 ₃ to the LDs 155 ₁-155₃, respectively, and instead send pilot signals to the LDs 155 ₁-155 ₃,which are transmitted upstream by the LDs 155 ₁-155 ₃ and received bythe TTVP 327. The TTVR 357 _(n) also sends a pilot signal to the LD 155_(n), which is transmitted upstream and received by the TTVP 327. Thetransmitted pilot signals are mutually orthogonal.

The TTVRs 357 ₁-357 _(n) and the TTVP 327 use the control channel tocoordinate and synchronize transmission of the pilot signals. The TTVR357 _(n) alerts the TTVP 327 that the CPE 350 _(n) wants to activate.The TTVP 327 then instructs the TTVR₁-TTVR_(n) to send the upstreampilot sequence. The TTVP 327 knows when the upstream pilot is to besent, and intercepts the resulting upstream signals.

In at least one example embodiment, a G.hn processing device, e.g. 125₁, sees a sequence of inserted zero values instead of a usual datasignal. For example, the controller 348 may transmit a command to theselector 332 c to indicate that channel estimation andprecoder/postcoder coefficient determinations are being performed. Basedon the command received from the controller 348, the selector 332 c maytransmit the zero values to the processing device 125 ₁ instead of theoutput from the postcoder 344. The reception of the zero values mayresult in a burst of errors. However, the processing device 125 ₁includes capabilities that allow recovery from error bursts, as are alsocaused, for example, by impulse noise.

In at least another example embodiment, the selectors 338 c, 338 d maynot be present or the controller 348 may instruct the selects 338 c, 338d to pass the output of the postcoder 344 to the processing device 1251.The G.hn protocol may be extended to be aware of the pilot sequencesand, for example, to temporarily stop sending and receiving data whenthe pilot sequence is being sent.

Since the CPE 350 _(n) requests to transmit data, the TTVP 327determines new upstream channel coefficients to include the channelsfrom the LDs 155 ₁-155 _(n) into the activating LD 130 _(n), and thechannels from the activating LD 155 _(n) into the LDs 130 ₁-130 _(n).Putting these new coefficients with the 3×3 channel matrix of previouslyactive lines creates a new 4×4 channel matrix.

Therefore, at S420, the TTVP 327 determines the postcoder coefficientsbased on the received pilot signals. For a system with n lines, theupstream resultant channel R_(u) at frequency f may be represented as ann by n matrix of the form:

R _(U) =C _(U)(I+G _(U))D _(U)  (3)

where C_(U) is an n by n postcoder matrix acting on frequency f, I is ann by n identity matrix, D_(U) is an n by n diagonal matrix representingthe direct gains in the upstream channel at frequency f, and G_(U) is ann by n normalized crosstalk channel matrix for frequency f.

The TTVP 327 may determine the upstream channel R_(u) as follows. Apilot sequence consists of S DMT symbols, where S is generally at leastas large as the number of lines in the system. Each symbol has a complexvalue at one of F frequencies, f=0, . . . , F−1. There is a pilotsequence associated with each line.

The complex value to be sent on the frequency f during symbol s on aline k is denoted by P_(k)(f,s).

The correlation between a symbol sequence x(s) and y(s) is defined as:

$\begin{matrix}{{\langle{{x(s)},{y(s)}}\rangle} = {\sum\limits_{s = 1}^{S}\; {{x(s)}\overset{\_}{y(s)}}}} & (4)\end{matrix}$

The pilot sequences for different lines k and j are mutually orthogonal,meaning that their correlation is zero, such that:

P _(k)(f,s),P _(j)(f,s)

=0  (5)

for all f and for any k not equal to j.

DMT modulation is used by the TTVP 327 and the TTVRs 357 ₁-357 _(n) toconvert the frequency-domain representation of a given pilot sequence tothe corresponding time-domain representation. That is, for a givensymbol s, the F complex frequency domain values P_(k)(f,s), f=0, . . . ,F−1, are transformed by Inverse FFT (IFFT) as is well-known in DMTmodulation to obtain 2F real, time-domain samples. These time-domainsamples are cyclically extended and windowed, as is well-known in DMTmodulation, to obtain T time domain samples, denoted p_(k)(t,s) for t=0,. . . , T−1.

To transmit a pilot sequence, a first TTVR (e.g., 357 ₁) first sends allof the time samples p_(k)(t, 1) corresponding to symbol 1, then all timesamples p_(k)(t,2) corresponding to symbol 2, and continues to transmitthe time samples for each symbol. The first TTVR may transmit thesymbols immediately one after the other, or they may be sentintermittently, with a number of normal upstream data samples from theCPE 350 ₁ sent in between pilot symbols. The TTVR pilot symbols may bematched to a FFT size used in the TTVP signal processing.

The TTVP 327 on line k receives time samples from the LD 130 ₁corresponding to the pilot symbol s. These time samples may be denotedas y_(k)(t,s). A subset of 2F consecutive time samples from each symbolmay be selected by the TTVP 327, as is well-known in DMT modulation, andtransformed by FFT to obtain F complex frequency domain values denotedY_(k)(f,s). After applying the postcoder 344, the resulting signals maybe denoted W_(k)(f,s). The TTVP 327 can perform signal processing on thereceived signals in order to estimate the resultant upstream crosstalkchannel coefficients (resulting from the concatenation of the crosstalkchannel and the postcoder 344). In particular, the TTVP 327 may estimatethe resultant upstream channel value in row v and column d by computing:

$\begin{matrix}{{{\hat{R}}_{v,d}(f)} = \frac{\langle{{W_{v}\left( {f,s} \right)},{P_{d}\left( {f,s} \right)}}\rangle}{\langle{{P_{d}\left( {f,s} \right)},{P_{d}\left( {f,s} \right)}}\rangle}} & (6)\end{matrix}$

In other words, the resultant upstream channel value in row v and columnd is determined by correlating the received, postcoded symbols on line vwith pilot symbols sent on the line d, and dividing by the result ofcorrelating the pilot symbols sent on the line d with themselves.

In one example embodiment, time-domain duplexing (TDD) is used to avoidnear-end crosstalk. That is, downstream and upstream transmissions arealternated in time. The physical layer FTTdp protocol is responsible forensuring TDD transmission of data signals. However, during the crosstalkestimation phases, the TTVP 327 and TTVRs 357 ₁-357 _(n) work togetherto ensure that the pilots are sent and received using TDD, regardless ofinput signals received from the processing devices 125 ₁-125 _(n) and160 ₁-160 _(n).

During all pilot sequence/crosstalk estimation phases in one exampleembodiment, the downstream output selectors 361 of all TTVRs 357 ₁-357_(n) and the upstream output selectors 332 c, 332 d of the TTVP 327 areall set to send zero values to the associated processing devices 125₁-125 _(n) and 160 ₁-160 _(n). This ensures that the higher layer is notdirectly affected by the pilot sequences—it is only affected by atemporary loss of signal.

The outputs of the TTVRs 357 ₁-357 _(n) and TTVP 327 that face the LDs130 ₁-130 _(n) and 155 ₁-155 _(n) are controlled as follows during thedifferent phases.

During the upstream estimation phase, TDD is enforced as follows. TheTTVR upstream output selectors 364 of all TTVRs 357 ₁-357 _(n) are setto send the upstream pilot sequences. The TTVP downstream outputselectors 332 a, 332 b for all lines are set to send zero values (sothat received pilot values are not corrupted by NEXT).

Referring back to FIG. 4B, in a system with 4 lines, where 3 lines areactive, and a fourth line about to initialize, C_(u) is a 4×4 postcoder,where the upper right 3×3 submatrix is a 3×3 postcoder, and where theremaining matrix elements are taken from the identity matrix I.

After determining the upstream channel R_(u), the TTVP 327 determinesnew postcoder coefficients to be implemented by the postcoder 344 asfollows:

C′ _(U)=(R _(U)diag(C _(U) ⁻¹ R _(U))⁻¹)⁻¹ C _(U)  (7)

Using the new postcoder coefficients C′_(U), the TTVP 327 cancelscrosstalk from the upstream channel, resulting in

R′ _(U) =D _(U)  (8)

In another example embodiment, the TTVP 327 may update the postcodercoefficients C_(u) implemented in the postcoder 344 using a first-orderapproximation to the channel inverse. Namely,

C′ _(U) =C _(U) +I−R _(U)diag(R _(U))⁻¹  (9)

This is also a reasonable embodiment requiring less computation, butwould not cancel the crosstalk as perfectly. An example embodiment ofintermediate complexity is

C′ _(U)=(2I−R _(U)diag(R _(U))⁻¹)C _(U)  (10)

Equations (7) and (10) relate to a method as taught in MultiplicativeUpdating Of Precoder Or Postcoder Matrices For Crosstalk Control In ACommunication System, U.S. patent application Ser. No. 13/016,376, theentire contents of which is incorporated by reference.

After the upstream channel is determined by the TTVP 327, the downstreamchannel is measured by the TTVP 327 using downstream pilots and upstreamloop-back. Referring back to FIG. 4A, the TTVP 327 transmits orthogonaldownstream pilot signals across the communication lines L1-Ln toassociated TTVRs 357 ₁-357 _(n) at S430, once the upstream channelR′_(U) is determined. At S430, the TTVP 327 ignores data being sent fromthe processing devices 125 ₁-125 _(n) and the TTVRs 357 ₁-357 _(n)ignore data from the processing devices 160 ₁-160 _(n).

With regards to upstream and downstream pilot sequences, in one exampleembodiment, the upstream and downstream pilot signals are designed sothat

|P _(k)(f,s)|² ≦M(f)  (11)

where M(f) specifies the maximum power for transmission at frequency f.The sum of |P_(k)(f,s)|² over all frequencies does not exceed any totalpower constraint. The peak to average ratio of p_(k)(t,s) allowshigh-fidelity modulation by the LDs (with little or no clipping).

For example, the pilot sequences may be based on a base symbol B(f) thatsatisfies the power constraints such that a corresponding time-domainrepresentation b(t) satisfies a peak-to-average ratio condition. Forexample, to achieve a good peak-to-average ratio condition with highprobability, the elements of B(s) might be 4-QAM values chosenindependently at random for each tone.

For each line k, a binary sequence q_(k)(s) is determined by the TTVP327 (i.e. q_(k)(s)=−1 or 1 for each s) such that the binary sequencesassigned to the different lines are mutually orthogonal. For example,the sequences might be obtained from rows of an S×S Walsh-Hadamardmatrix.

The pilot sequences are constructed by the TTVP 327 using outer productsof the base symbols with the binary sequences, as

P _(k)(f,s)=B(f)q _(k)(s)  (12)

At S430, the TTVR upstream output selectors 364 of all TTVRs 357 ₁-357_(n) are set to send zero values. The TTVP downstream output selectors332 a, 332 b for all lines are set to send the downstream pilotsequences. During this part, the received values are buffered by theTTVRs 357 ₁-357 _(n) in the amplification, filtering and delay unit 362.

At S440, each TTVR 357 ₁-357 _(n) loops back each pilot signal that isreceived. In other words, each TTVR 357 ₁-357 _(n) retransmits thereceived pilot signals in the upstream direction. In some exampleembodiments, each TTVR 357 ₁-357 _(n) may filter and amplify the pilotsignals that are received and then retransmit the pilot signal.

The TTVR upstream output selectors 364 of all TTVRs 357 ₁-357 _(n) areset to the amplified, filtered, and delayed signal stored in theamplification, filtering and delay unit 362. The TTVP downstream outputselectors 332 a, 332 b for all lines are set to send zero values (sothat received pilot values are not corrupted by NEXT).

The amplification used in the loop-back can be done in many ways by theTTVRs 357 ₁-357 _(n). For example, a diagonal amplification matrix L(f)may be implemented without violating power constraints at the CPEs 350₁-350 _(n).

In different example embodiments, the filtering and amplificationparameters are specified by the CPEs 350 ₁-350 _(n), based on knowledgeof the upstream channel. Or, the parameters are determined by the CPEs350 ₁-350 _(n) based on properties of the received downstream pilotsignals.

Also at S440, the distribution point 310 receives loop-backed pilotsignals. Upon receiving the loop-backed pilot signals, the TTVP 327determines the loop-back channel coefficients. The loop-back channel isconsidered to be formed from the concatenation of the downstreamchannel, the amplification filter (if any), and the upstream channel.

The loop-back channel may defined as:

R _(L) =C′ _(U)(I+G _(U))D _(U) LD _(D)(I+G _(D))C _(D) =D _(U) LD_(D)(I+G _(D))C _(D)  (13)

For the downstream, loop-back case, the pilot sequences can be definedin the same way as described above, but the time domain pilotsp_(k)(t,s) are conveyed from the TTVP 327 to the LDs 130 k. Signals arelooped back upstream by the TTVRs 357 ₁-357 _(n), and then y_(k)(t,s)represents the resulting time-domain values sent back from the LDs 130₁-130 _(n) to the TTVP 327. The TTVP 327 processes the values y_(k)(t,s)as above to obtain postcoded symbols W_(k)(f,s), which are correlatedwith pilot symbols as above in Equation (6) to obtain estimates of theresultant loop-back channel value in row v and column d.

At S440, the TTVP 327 receives the result of sending the pilot signalsthrough the loop-back channel R_(L). These received signals, togetherwith the knowledge of the transmitted pilots, can be used in S450 todetermine the precoder coefficients. Based on knowledge of R_(L) andC_(D), the TTVP 327 generates new precoder coefficients

C′ _(D) =C _(D)(diag(R _(L) C _(D) ⁻¹)⁻¹ R _(L))⁻¹  (14)

which diagonalize the loop-back channel, resulting in

R′ _(L) =D _(U) LD _(D)  (15)

where L is the diagonal matrix with i-th diagonal elements being theloop-back amplification applied by the TTVR on communication line i. Inanother example embodiment, the TTVP 327 may update the coefficientsimplemented in the precoder 336 using a first-order approximation to thechannel inverse. Namely,

C′ _(D) =C _(D) +I−diag(R _(L))⁻¹ R _(L)  (16)

Another example embodiment of intermediate complexity is

C′ _(D) =C _(D)(2I−diag(R _(L))⁻¹ R _(L))  (17)

The downstream channel is also diagonalized

R′ _(D) =D _(D)(I+G _(D))C′ _(D) =D _(D)  (18)

As a result, the concatenation of the updated precoder 336 and thedownstream channel provides a nearly crosstalk-free resultant channelfrom processing devices 125 ₁-125 _(n) to processing devices 160 ₁-160_(n), and the concatenation of the upstream channel and the updatedpostcoder provides a nearly crosstalk-free resultant channel fromprocessing devices 160 ₁-160 _(n) to processing devices 125 ₁-125 _(n).An active communication session can then be established on the line Ln,as shown in FIG. 4C, without affecting existing active sessions onL1-L3. The above discussion was given in the context of a singlenarrowband channel at frequency f. The method also works when there aremultiple channels at multiple frequencies, for example in a discretemulti-tone system. Each channel matrix previously described then becomesa matrix-valued function of frequency, e.g., R_(U)(f).

FIG. 5 illustrates a method of reducing crosstalk using a pilotloop-back with NEXT and echo cancellation. The method shown in FIG. 5may be performed by at least the TTVP shown in FIG. 3E and the TTVRshown in FIG. 3F. In FIG. 5, the TTVP receives upstream pilots at S510,determines postcoder coefficients at S520, transmits downstream pilotsat S530, determines NEXT canceller coefficients at S540, transmitsdownstream pilots and receives loopback pilots at S545 and determinesprecoder coefficients at S550.

Steps S500, S510, S520 and S550 are the same or substantially the sameas the steps S400, S410, S420 and S450, respectively, shown in FIG. 4A.Therefore, steps S500, S510, S520 and S550 will not be described ingreater detail for the sake of brevity.

At S530, the TTVP transmits downstream pilots to determine the NEXTcanceller coefficients at S540. The NEXT canceller coefficients may bedetermined using any known method. Moreover, at S530, a TTVR maydetermine the coefficients implemented in the echo canceller.

In FIG. 5, the steps S430 and S440, of FIG. 4, can be executedsimultaneously, instead of sequentially. Therefore, at S545, the TTVPtransmits downstream pilots and receives loopback pilot signalssimultaneously. For example, information may travel down the line in ashorter time than the time a pilot symbol or an entire pilot sequence issent. Since the NEXT canceller is trained at S540, and the echocanceller is trained at S530, then full-duplex can be used and thesignals sent upstream and downstream don't interfere with each other.Therefore, while the TTVP is partway through sending the pilot sequence,the TTVR receives the first values of the pilot sequence. In full-duplexmode, the TTVR starts sending received values of the downstream pilotsupstream (the loopback) as soon as the downstream pilots are received,without waiting for the TTVP to finish sending the last values of thepilot sequence.

As described, the methods of FIGS. 4A and 5 allow estimation of adownstream MIMO (multiple input, multiple output) channel withoutrequiring digital upstream communication sessions. Consequently, CPEcomponents designed for SISO (single input, single output)communications may be used while building a MIMO system capable ofcancelling crosstalk and hence achieving significantly higher datarates.

Compared with conventional methods of downstream estimation, exampleembodiments facilitate taking customer-side transceiver hardware andsoftware modules designed for SISO channels and re-using them in a MIMOcontext, while achieving crosstalk cancellation.

In particular, conventional methods are for the CPE to provide feedbackto the operator side over an upstream digital communication channel. Thefeedback can be in the form of quantized estimates of downstream channelparameters, or quantized measurements of received pilot signals, errorsignals, and the like. The G.vector standard can be taken as arepresentative example of a conventional approach.

For example, changes introduced by MIMO (G.vector) relative to SISO(VDSL2) include normal SISO initialization procedure is interrupted atleast once for a MIMO estimation phase. This is because someinitialization is required to establish a basic communication channelupstream, necessary for crosstalk estimation. However, theinitialization cannot be optimized until after crosstalk estimation iscomplete. Channels for upstream feedback of training information have tobe established. In the case of G.vector, there is a special channelcreated for feedback during initialization, and another mechanismdefined for feedback during ordinary operation. Error measurements orcrosstalk estimates need to be collected and then sent on the feedbackchannel. This process needs to be configured and managed.

Consequently, the digital feedback in conventional methods requiressignificant changes to software, firmware, and or hardware, relative toa system that had only to deal with SISO.

By contrast, in example embodiments, the MIMO functionality is separatedfrom the SISO functionality. In terms of the signal path, the loop-backoccurs early in the processing chain, immediately after the analog todigital conversion. Hence circuits involved in providing feedback can bephysically or logically separated from the remaining SISO components.

In terms of time, the MIMO training can all be done before the ordinarySISO training procedure. Hence, there is little or no need to change theSISO training software or firmware—the MIMO channel is diagonalized(virtually creating multiple SISO channels) before the SISOinitialization process starts.

As described above, example embodiments provide for a transparent methodof reducing and/or eliminating crosstalk with short distances (e.g.,less than 200 m). Since distribution point is only 200 m away, a homenetwork G.hn processing device may be used in systems that reducecrosstalk. Also, using G.hn processing devices instead of specializedprocessing devices reduces the cost.

Example embodiments being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of example embodiments, and allsuch modifications as would be obvious to one skilled in the art areintended to be included within the scope of the claims.

1. A method of determining crosstalk in a multiple-input-multiple output(MIMO) system, the method comprising: receiving, from at least one firstremote node, upstream pilots on an upstream channel; determiningupstream channel coefficients based on the received pilots;transmitting, to the at least one first remote node, downstream pilotson a downstream channel; receiving, from the at least one first remotenode, loopback pilots on the upstream channel, the loopback pilots beingloopback signals of the downstream pilots; and determining downstreamchannel coefficients based on the received downstream pilots.
 2. Themethod of claim 1, wherein the upstream pilots are mutually orthogonalto upstream pilots received on other upstream channels.
 3. The method ofclaim 1, wherein the transmitting the downstream pilots transmits thedownstream pilots after receiving the upstream pilots.
 4. The method ofclaim 1, wherein the downstream pilots are mutually orthogonal.
 5. Themethod of claim 1, wherein the loopback pilots are amplified by the atleast one remote node.
 6. The method of claim 5, wherein the loopbackpilots are at least one of filtered and delayed by the at least oneremote node.
 7. The method of claim 1, further comprising: precodingdata based on the downstream channel coefficients; and transmitting theprecoded data to the remote node.
 8. The method of claim 1, furthercomprising: receiving data from the at least one first remote node; andpostcoding the data based on the upstream channel coefficients.
 9. Themethod of claim 1, wherein the receiving upstream pilots includes,receiving a request to transmit data from one of the at least one firstremote node, and receiving the upstream pilots from the at least onefirst remote node and the other first remote nodes.
 10. The method ofclaim 9, wherein the determining upstream channel coefficients includes,determining the upstream channel coefficients based on previous upstreamchannel coefficients, the previous upstream channel coefficients beingchannel coefficients before receiving the request to transmit data. 11.The method of claim 1, further comprising: precoding data based on thedownstream channel coefficients; and transmitting the precoded data tothe at least one first remote node.
 12. The method of claim 1, furthercomprising: receiving data from the at least one first remote node; andpostcoding the data based on the upstream channel coefficients.
 13. Aprocessor configured to, receive, from at least one first remote node,upstream pilots on an upstream channel; determine upstream channelcoefficients based on the received pilots; transmit, to the at least onefirst remote node, downstream pilots on a downstream channel; receive,from the at least one first remote node, loopback pilots on the upstreamchannel, the loopback pilots being loopback signals of the downstreampilots; and determine downstream channel coefficients based on thereceived downstream pilots.
 14. The processor of claim 13, wherein theprocessor is configured to transmit the downstream pilots orthogonally.15. The processor of claim 13, wherein the processor is configured to,precode data based on the downstream channel coefficients; and transmitthe precoded data to the remote node.
 16. The processor of claim 13,wherein the processor is configured to, receive data from the at leastone first remote node; and postcode the data based on the upstreamchannel coefficients.
 17. The processor of claim 13, wherein theprocessor is configured to, receive a request to transmit data from oneof the at least one first remote node, and receive the upstream pilotsfrom the at least one first remote node and the other first remotenodes.
 18. The processor of claim 17, wherein the processor isconfigured to, determine the upstream channel coefficients based onprevious upstream channel coefficients, the previous upstream channelcoefficients being channel coefficients before receiving the request totransmit data.